1. Field
The present disclosure relates to a method and a system for identifying the edges of web media transported on a movable transport surface.
2. Description of Related Art
A full width array sensor is used for monitoring or controlling several sub-systems in different image printing systems. For example, the full width array sensor is used for uniformity correction as well as jet forming and registration. In many of these image printing systems, the sensor is calibrated at regular intervals to ensure a uniform response. The full width array sensors are calibrated by measuring the response of each sensor element in the absence of light and the response of each sensor element to a uniform exposure. The latter measurement is typically made using a white calibration strip that is known to have a uniform reflectivity across its surface. From the calibration, the relative light measured by each sensor element in the full width array sensor can be determined independent of the sensor's offset (i.e., dark level response of each sensor element) and gain (i.e., sensitivity of the sensor element to light).
In a continuous feed direct marking printer, the standard approach to the full width array sensor calibration is difficult. The full width array sensor is fixed in place where the sensor views the web media as the web media passes under the sensor. Creating an architecture where the full width array sensor moves to measure a calibration strip is generally not preferred. Therefore, the blank media itself is generally used as the calibration strip.
The web media passes over a roller which ensures that the spacing between the web media and the full width array sensor remains fixed and thus the image remains in focus. The web media is illuminated by a light source and the reflected light is measured by the full width array sensor. For thin web media, some portion of the light passes through the web media and is reflected by the roller. The amount of light passing through the web media depends on the local thickness of the web media. To ensure that variations in the local thickness of the web media do not add noise to a measurement of the uniformity, a white backer roller is generally used.
In general, the reflectance of the backer roll may differ slightly from the reflectance of the web media. However, the calibration of the sensor eliminates the ability to monitor this difference. For paper edge detection, the full width array sensor is generally wider than the web media. Some sensors monitor/view the web media and other sensors monitor/view the roller. The calibration process forces the full width array sensors that monitor/view the roller to have an equal response to those sensors that monitor/view the web media, providing no contrast across the transition from the web media to the roller. This means that the reflectivity difference between the backer roll and the paper may not be used to discriminate between the backer roll and the paper.
The present disclosure provides improvements over the prior art.